Porous material of polytetrafluoroethylene and production process thereof

ABSTRACT

A porous material of a polytetrafluoroethylene (PTFE), which can retain at least 90% of 0.109 μm latex spheres and has a flow rate (IPA flow rate) of at least 0.6 ml/cm 2  /min as measured with isopropyl alcohol under a differential pressure of 1 kg/cm 2 . A process for the production of a porous material of PTFE, which comprises, after forming a molded article from fine powder of PTFE by paste extrusion, (1) sintering the molded article, (2) rolling up the molded article thus sintered and heat-treating it at a temperature lower than the melting point of the polytetrafluoroethylene in a state as rolled up, and then (3) stretching the molded article thus heat-treated in at least an uniaxial direction. The porous material of PTFE according to the present invention has micropores and good permeability and particularly is excellent in performance of retaining fine particles.

This application is a 371 application of PCT/JP94/00735 filed May 2,1994.

TECHNICAL FIELD

The present invention relates to a porous material of apolytetrafluoroethylene and a production process thereof, and moreparticularly to a porous material of a polytetrafluoroethylene, whichhas micropores, and is high in porosity and excellent in permeability,and a production process thereof.

BACKGROUND ART

Porous materials comprising a polytetrafluoroethylene (hereinafterabbreviated as "PTFE") as a material are used in a wide variety offields such as separator for cells, membrane filters, electric wires,analytical instruments and artificial blood vessels. Now, in recentyears, porous materials of PTFE having a minute pore diameter andexcellent permeability have been required of application fields such asprecision filters, high-performance separation membranes and artificiallung septa. Therefore, porous materials of PTFE having micropores and ahigh porosity have been required. However, it has been very difficult toproduce a porous material of PTFE, which combines micropores with a highporosity and has excellent permeability.

As a process for producing a porous material of PTFE, it has heretoforebeen known to stretch an unsintered molded article obtained by pasteextrusion of PTFE at a temperature not higher than the melting point ofPTFE and then sinter the molded article (Japanese Patent Publication No.13560/1967). According to this process of stretching the unsinteredmolded article, porous material of PTFE having various porosities can beobtained. However, the pore diameter becomes greater as the draw ratiois increased to enhance the porosity. Therefore, there has been a limitin the production of porous material of PTFE combining micropores and ahigh porosity.

As another process for producing a porous material of PTFE, it has alsobeen proposed to heat a molded article of PTFE to a temperature notlower than 327° C., slowly cool the sintered molded article so as toheat-treat it to give a crystallinity of 80%, and then uniaxiallystretch the sintered molded article at a draw ratio of 1.5-4 times at atemperature of 25-260° C. (Japanese Patent Publication No. 42794/1978).According to this process (hereinafter abbreviated as "the slow coolingprocess"), a porous material of PTFE in which micropores have beenformed can be obtained. In the slow cooling process, however,crystallization is not allowed to fully progress if the cooling rate istoo fast in the step of slowly cooling the sintered molded article ofPTFE. It is thus necessary to decelerate the cooling rate. Accordingly,this process has involved a problem that precise temperature control andlarge equipment are required.

More specifically, it is said in the slow cooling process that it ispreferable to cool the sintered molded article generally at a rateslower than about 0.5° C./min for enhancing the crystallinity of theresulting sintered molded article of PTFE. In Examples of thispublication, cooling rates of 0.25° C./min, 0.1° C./min and 0.05° C./minare used. In order to perform slow cooling at such a low cooling rate,it is necessary to conduct temperature control with extremely highprecision. In addition, porous materials of PTFE are generally formed ascontinuous molded articles such as rods, tubes, strips and sheets bypaste extrusion of fine powder of PTFE. The PTFE porous material isformed through a heat-treating step, a stretching step and the like. Itis however difficult and impracticable to apply the slow cooling processto these continuous sintered molded articles. For example, in order tocool a sintered molded article in the form of a continuous sheet from350° C. to 290° C. at a cooling rate of 0.5° C./min by means of an oven3 m long, it is necessary to pass the sheet through the oven over 2hours. The transit time in the oven is 1.5 m/hr in terms of linearvelocity. Therefore, in the case where the length of the continuoussheet is 100 m, it takes about 67 hours to pass through the oven. On theother hand, in order to slowly cool a continuous sheet 100 m long in 20hours under the above-described cooling conditions, it is necessary topass the sheet through the oven at a linear velocity of 5 m/hr.Therefore, a large oven as long as 10 m is required.

As described above, in the process in which the sintered molded articleis slowly cooled from the temperature not lower than the melting pointof PTFE, the production of a continuous sintered molded article requireseither a very long oven or a treatment at a very low linear velocity.Therefore, there is a limit in industrial practice.

Japanese Patent Application Laid-Open No. 78823/1989 discloses aproduction process of a porous PTFE membrane, in which fine powder of aPTFE having a number average molecular weight of 1,000,000 or lower ispaste-extruded into a molded article, the molded article is sintered,the sintered molded article is slowly cooled from the sinteringtemperature at a rate lower than 10° C./hr (1° C./hr in Example 1) toenhance its crystallinity, and the thus-cooled sintered molded articleis then stretched in at least an uniaxial direction. One of theco-inventors of the present invention proposed a process in which acontinuous molded article of PTFE is sintered, and the resultantsintered molded article is slowly cooled while passing the sinteredmolded article through at least two different zones, which aresuccessively preset from a higher-temperature region to alower-temperature region in a temperature range of 350-290° C. and arecontrolled at substantially fixed temperatures, thereby enhancing itscrystallinity, and previously applied for a patent (Japanese patentApplication Laid-Open No. 8344/1994). When the molded articles enhancedin crystallinity according to these processes are stretched, porous PTFEmembranes having micropores and a high porosity can be obtained.However, the molded articles enhanced in crystallinity according tothese processes tend to break if they are stretched at a draw ratio of10 times or higher. As a result, the porosities of the resulting porousPTFE membranes have been at most 65% or so. The reason for it isconsidered to be attributable to the fact that in these processes forenhancing crystallinity, the molded article is held for a considerablylong period of time at a temperature not lower than the melting point ofPTFE, and so microstructural thermal decomposition occurs, therebyreducing elongation percentage.

A PTFE filter is excellent in heat resistance and chemical resistanceand hence used mainly in filtration of chemicals and gases in a field ofsemiconductors. With the high integration of semiconductors in the fieldof semiconductors, there is a strong demand for development of a PTFEfilter having a minuter pore diameter. Since the yield ofhigh-integrated semiconductors is affected by the retention of a PTFEfilter, there is a demand for development of a filter high in theretention of fine particles. Namely, judging from the recent performancerequirement for PTFE filters, it is desirable that the 0.109 μm latexretention be at least 90%, preferably at least 99%, more preferably100%. In commercially-available PTFE filters (pore diameter: 0.1 μm and0.05 μm), however, the 0.109 μm latex retention is up to a maximum ofabout 70%. On the other hand, it has been known a porous PTFE membranehaving a pore diameter of 0.02 μm. However, its flow rate (IPA flowrate) as determined with isopropyl alcohol is as extremely low as 0.0005ml/cm² /min (as measured under a differential pressure of 0.95 kg/cm²),and so the filter is lacking in practical performance.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a porous material ofa polytetrafluoroethylene, which combines micropores with a highporosity and has excellent permeability, and a production processthereof.

The present inventors have carried out an extensive investigation with aview toward developing a process for producing a porous material of PTFEhaving micropores and a high porosity with ease even if it is acontinuous molded article such as a continuous sheet. As a result, ithas been found that such an object can be achieved by sintering a moldedarticle obtained by paste extrusion of PTFE fine powder, rolling up themolded article, heat-treating the molded article at a temperature lowerthan the melting point of PTFE in a state as rolled up, and thenstretching the molded article thus heat-treated in at least an uniaxialdirection.

In the conventional slow cooling process, its application to acontinuous molded article requires an elaborate equipment for conductingprecise temperature control and control of cooling rate. According tothe process of the present invention, however, a porous material of PTFEhaving micropores and a high porosity can be obtained by applying a verysimple heat-treating process, in which the sintered molded article isrolled up around a roll or the like and then held for a certain periodof time at a temperature lower than the melting point of PTFE.

According to the process of the present invention, the crystallizationof the molded article is advanced by a substantially isothermal heattreatment at a temperature lower than the melting point of PTFE.Therefore, elongation percentage is also enhanced, and so the moldedarticle can be stretched up to about 30 times, and the porosity of theporous material of PTFE can be increased from a maximum of about 65% inthe conventional porous materials of PTFE to a maximum of about 90%.

In addition, the porous material of PTFE according to the presentinvention can retain at least 90% of 0.109 μm latex spheres, preferablyat least 99%, more preferably 100%. Besides, the porous material of PTFEaccording to the present invention has an IPA flow rate of at least 0.6ml/cm² /min and is hence excellent in practical performance as a filter.

The present invention has been led to completion on the basis of thesefindings.

According to the present invention, there is thus provided a porousmaterial of a polytetrafluoroethylene, which can retain at least 90% of0.109 μm latex spheres and has a flow rate (IPA flow rate) of at least0.6 ml/cm² /min as measured with isopropyl alcohol under a differentialpressure of 1 kg/cm²,

wherein the retension rate is determined in the following manner:

A disk 47 mm across is punched out of a sample membrane and set in aholder. Then, 32 cm³ of an aqueous solution containing uniform particlesof a polystyrene latex sphere having a particle diameter of 0.109 μm ina concentration of 1.4×10¹⁰ particles/cm³ is filtered through the diskunder a pressure of 0.42 kg/cm². At this time, the retention of theparticles is determined by means of a spectrophotometer for ultravioletand visible region at a wavelength of 310 nm.

According to the present invention, there is also provided a process forthe production of a porous material of a polytetrafluoroethylene, whichcomprises, after forming a molded article from fine powder of thepolytetrafluoroethylene by paste extrusion, (1) sintering the moldedarticle, (2) rolling up the molded article thus sintered andheat-treating it at a temperature lower than the melting point of thepolytetrafluoroethylene in a state as rolled up, and then (3) stretchingthe molded article thus heat-treated in at least an uniaxial direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning microphotograph (5,000 magnifications) showing afine fibrous structure of a porous PTFE sheet according to the presentinvention, which was obtained in Example 3. A drawing under thephotograph indicates that the upper left part of the photograph is asurface, and the lower part thereof is a section.

FIG. 2 is a scanning microphotograph (5,000 magnifications) showing afine fibrous structure of a porous PTFE sheet according to the priorart, which was obtained in Comparative Example 3. A drawing under thephotograph indicates that the upper left part of the photograph is asurface, and the lower part thereof is a section.

BEST MODE FOR CARRYING OUT THE INVENTION

Fine Powder of Polytetrafluoroethylene

PTFE used in the present invention is in the form of fine powder. Thenumber average molecular weight of PTFE ranges from hundreds ofthousands to tens of millions, and no particular limitation is imposedon the molecular weight. It is however preferable to use a PTFE having arelatively low molecular weight for applying the production process ofthe present invention thereto. The reason for it is that theheat-treating time required of a PTFE having a lower molecular weightcan be shortened compared with a PTFE having a higher molecular weight,and so a porous material of PTFE having micropores and a high porositycan be obtained in a relatively short period of time. As the PTFE havinga low molecular weight, those having a molecular weight not higher than2,000,000 are preferred. A particularly preferred range of the molecularweight is from 200,000 to 2,000,000.

Paste Extrusion

The first step of producing the porous material of PTFE according to theprocess of the present invention is formation of a molded article by apaste extrusion process heretofore known as a production process for anunsintered sheet or the like. In the step of the paste extrusion, aliquid lubricant is incorporated in PTFE in a proportion of 15-40 partsby weight, preferably 20-30 parts by weight per 100 parts by weight ofPTFE to conduct extrusion.

As the liquid lubricant, may be used various lubricants heretofore usedin the paste extrusion process. Specific examples thereof includepetroleum solvents, hydrocarbon oils such as solvent naphtha and whiteoil, aromatic hydrocarbons such as toluol and xylol, alcohols, ketones,esters, silicone oil, fluorocarbon oil, solutions obtained by dissolvinga polymer such as polyisobutylene or polyisoprene in these solvents,mixtures of two or more these lubricants, and water or aqueous solutioncontaining a surfactant.

The formation by the paste extrusion is performed at a temperature lowerthan the melting temperature (327° C.) of PTFE, generally, a temperaturenear room temperature. Preforming is generally conducted prior to thepaste extrusion. In general, a mixture of PTFE and the liquid lubricantis preformed under a pressure of, for example, 1-50 kg/cm² or so, andthe preformed article is then extruded through a paste extruder orrolled by calender rolls or the like, or extruded and then rolled into amolded article in the desired shape.

Examples of the shape of the molded article obtained by the pasteextrusion include various shapes such as a rod, tube, strip and sheet. Athinner sheet can also be obtained by further rolling the sheet thusformed. The molded article according to the present invention may haveany shape so far as it can be subjected to stretching after itssintering.

The liquid lubricant is removed from the molded article obtained by thepaste extrusion by heating, extraction or dissolution prior to thesintering of the molded article. The heating temperature in this case isgenerally 100-300° C. However, the removal by extraction may preferablybe adopted if a liquid lubricant having a relatively high boiling point,such as silicone oil or a fluorocarbon, is used.

By the way, other substances in addition to the liquid lubricant may beincorporated as necessary for the intended application. For example, aspigments for coloring, and for the purpose of improvement of abrasionresistance, prevention of cold flowand easy formation of pores, carbonblack, graphite, silica powder, asbestos powder, glass powder, glassfibers, inorganic fillers such as silicates and carbonates, metalpowders, metal oxide powders, metal sulfide powders may be added to themixture. In order to facilitate the formation of a porous structure,substances capable of being removed or decomposed by heating, extractionor dissolution, for example, ammonium chloride, sodium chloride, otherplastics, rubbers and the like may also be incorporated in the form ofpowder or a solution.

Sintering

The molded article obtained by the paste extrusion is then sintered. Thesintering is conducted by heating it at a temperature not lower than thetransition point (327° C.) of PTFE for several minutes to several tensminutes, or a longer period of time if circumstances require. Thesintering is generally performed by passing the molded article through aheating oven controlled at 350-500° C.

Heat Treatment

In the present invention, the sintered molded article of PTFE is rolledup around a roll or the like after the sintering and then subjected to aheat treatment in a state as rolled up, namely, in a constrained state.The heat treatment is conducted at a temperature lower than thetransition point (melting point) of PTFE under conditions that thecontinuous sintered molded article rolled up, such as a sheet, is notfusion-bonded itself.

The heat-treating temperature is preferably within a range of 280-325°C., more preferably 300-320° C. The heat treatment can be carried out byholding the sintered molded article of PTFE as rolled up in an oven keptat a predetermined temperature. The retention time in the oven isgenerally 1-50 hours, preferably 5-30 hours.

By conducting this heat treatment, the molecule of PTFE is orientated,and so the crystallinity of PTFE is increased. It is preferable toincrease the crystallinity of the sintered molded article to at least80% from the viewpoint of the formation of micropores and theenhancement of porosity. In order to obtain a porous material of PTFEhaving micropores and a high porosity, it is preferable to conduct theheat treatment over 5 hours or longer in the above-described temperaturerange. However, it is preferable from the viewpoint of efficiency todetermine the upper limit of the heat-treating time within preferably 50hours, more preferably 30 hours.

In the heat-treating step according to the present invention, the heattreatment of the sintered molded article of PTFE is performed in a stateas rolled up at a temperature lower than the melting point of PTFE.Therefore, a number of sintered molded articles can be heat-treated atthe same time in an oven. A porous material having a higher porosity isprovided by stretching the sintered molded article in a subsequentstretching step as the crystallinity of the sintered molded articlecrystallized increases. The stretchability of the sintered moldedarticle itself is also more improved as the crystallinity increases.

The production process of the present invention has the greatest featurein that the heat-treating step for enhancing the crystallinity isperformed after the non-sintered molded article is sintered and thenrolled up. The sintered molded article of PTFE can be rolled up, and thefusion bonding of the rolled article itself can be prevented so far asit is heat-treated at a temperature lower than the melting point ofPTFE. The heat treatment can be conducted by holding the sintered moldedarticle in a state as rolled up in an oven controlled at a predeterminedtemperature without need of conducting slow cooling like theconventional processes.

According to the production process of the present invention, the heattreatment for enhancing the crystallinity of the sintered molded articleis performed at a temperature lower than the melting point of PTFE undersubstantially isothermal conditions. Therefore, microstructural thermaldecomposition can be avoided. Accordingly, since the sintered moldedarticle subjected to the heat treatment advances in crystallization andits elongation percentage increases, it is not broken even when it isstretched at a draw ratio as high as 30 times or so.

After the heat treatment, the sintered molded article can be unwound tofeed to a subsequent stretching step. According to the process of thepresent invention, therefore, a crystallization treatment of acontinuous sintered molded article such as a continuous sheet, which hasheretofore been difficult to carry out, can be performed with ease.

Stretching

The sintered molded article obtained by the crystallization treatmentand having high crystallinity is then stretched in at least an uniaxialdirection. The stretching can be performed by mechanically stretching anarticle in the form of a sheet, rod, tube or the like by a method knownper se in the art. For example, in the case of a sheet, it can bestretched by holding its two sides opposite to each other and stretchingit so as to widen its space, or by making a take-up rate higher than afeed rate upon rolling up the sheet from one core to the other core. Inthe case of a rod or tube, it is simple to stretch it in its lengthwisedirection. Biaxial stretching may also be conducted successively orsimultaneously.

The stretching is performed at a temperature lower than the meltingpoint of PTFE, generally, 0-260° C. The stretching at a lowertemperature tends to form a porous material relatively great in porediameter and high in porosity. The stretching at a higher temperaturetends to form a close porous material relatively small in pore diameter.The porosity of the porous material increases as the draw ratio ishigher. Therefore, a porous material having the desired physicalproperties can be obtained by suitably combining these conditions witheach other.

In the stretching step, the porosity of the resulting porous material ofPTFE increases as the draw ratio is made higher. In order to obtain aporous material high in porosity and excellent in permeability,therefore, it is desirable to perform the stretching at a draw ratio ofat least 5 times (area ratio), preferably 6-30 times or so, morepreferably 9-30 times or so. In the case where biaxial stretching isconducted, it is preferable to stretch the article in one direction at adraw ratio of 2 times to 8 times or so and control a ratio of thestretching in a longitudinal direction to the stretching in a transversedirection within a range of from 1:5 to 5:1. It is also permissible toconduct the first-stage stretching at a low temperature of 20° C. or soand then perform the second-stage stretching under higher-temperatureconditions.

The porous material of PTFE obtained by the stretching tends to shrinkwhen it is heated to 327° C., which is a melting point of PTFE, orhigher. In addition, when the porous material thus stretched is left tostand without setting, it shrinks to lose its porous structure or tocause irregularities of the porous structure. Therefore, it ispreferable to conduct heat setting after the stretching. The heatsetting is conducted by holding the stretched article at a temperatureof 150-250° C. or so for 1-30 minutes or so while keeping the stretchedstate under tension by the fixation of both ends, or the like.

Porous Material of Polytetrafluoroethylene

The porous material of PTFE according to the present invention can haveany shape, for example, a shape of a sheet or tube, according to theform of the molded article obtained by the paste extrusion, and has afeature in that it combines micropores with a high porosity.Specifically, the porous material of PTFE according to the presentinvention has the following properties:

(1) The pore diameter of the porous material of PTFE varies according tothe crystallinity, draw ratio and the like of the sintered article ofPTFE.

(2) According to the present invention, since the draw ratio can be madehigher than that of the conventional process, the porous material can bemade microporous, and its porosity (as determined in accordance withASTM-D-792) can be made as high as 60-90% or so, preferably 70-90% orso, more preferably 80-90% or so.

(3) With respect to the thickness of the porous material of PTFE, porousmaterials having various thicknesses can be produced by changing thedraw ratio or the like. Thin membranes having a thickness of 50 μm orthinner, moreover 10 μm or so can be obtained with ease by increasingthe draw ratio.

(4) The bubble point (as determined with isopropyl alcohol in accordancewith ASTM-F-316-76) of the porous material of PTFE according to thepresent invention is generally 2-8 kg/cm² or so.

(5) The IPA flow rate (as determined with isopropyl alcohol under adifferential pressure of 1 kg/cm²) of the porous material of PTFEaccording to the present invention is generally at least 0.6 ml/cm²/min.

(6) The porous material of PTFE according to the present invention isfar excellent in retention performance. In the case of a filtrationmembrane, it can retain at least 90% of 0.109 μm latex spheres,preferably at least 99%, more preferably 100%.

On the other hand, commercially-available porous PTFE membranesseparately having pore diameters of 0.1 μm and 0.05 μm have retention ofparticles having a particle diameter of 0.109 μm of about 10% and about70%, respectively. A porous PTFE membrane having a pore diameter of 0.02μm is also commercially available. However, its IPA flow rate is asextremely low as 0.0005 ml/cm² /min (under a differential pressure of0.95 kg/cm²), and so the retention of the particles cannot bedetermined. Besides, the porous material of PTFE according to thepresent invention can retain at least 30% of 0.073 μm latex spheres,preferably at least 50%, more preferably at least 60%.

(7) The porous material of PTFE according to the present invention alsohas a feature in a microstructure. FIG. 1 is a photograph (5,000magnifications), taken by a scanning electron microscope (SEM), of aporous PTFE sheet obtained in Example 3 according to the presentinvention. The upper left part of the photograph is a surface, and thelower part thereof is a section. According to the production process ofthe present invention, the molded article of PTFE produced by the pasteextrusion of fine powder of PTFE is first sintered. At this time, thePTFE resin melts. When the sintered molded article is then rolled up andthen heat-treated at a temperature lower than the melting point of PTFE,the sintered molded article subjected to the heat treatment advances incrystallization. It is considered that the resin melted upon thiscrystallization is reorganized, particularly, in a thickness direction.When the sintered molded article to which high crystallinity has beenimparted in this manner is stretched at a high draw ratio, a structurein which a fine fibrous structure is three-dimensionally networked isformed. The sizes of pores formed in the surface and the section of theporous PTFE sheet are substantially the same.

On the other hand, a porous PTFE sheet according to Comparative Example3, obtained in accordance with the conventional process in which anunsintered molded article obtained by the paste extrusion of PTFE isstretched at a temperature not higher than the melting point of PTFE andthen heat-treated (for example, Japanese Patent Publication No.13560/1967), is close only in the surface thereof (the same may be saidof the back side) and coarse in the interior thereof and forms layers asillustrated by an SEM photograph (5,000 magnifications) in FIG. 2.

By such a structural difference, the porous PTFE sheets in FIGS. 1 and 2have a marked difference in the retention of a particle diameter of0.109 μm though their bubble points are 3.8 kg/cm² and the same. Theparticles can be retained by 100% by the invention product (Example 3),but only by 55% by the comparative product (Comparative Example 3).

By the way, the porous PTFE membranes obtained by stretching articles ofthe crystallinity of which has been enhanced by processes described inJapanese Patent Application Laid-Open No. 78823/1989 and Japanese PatentApplication No. 188613/1992 cannot have such a three-dimensional networkstructure as illustrated in FIG. 1 because they cannot be stretched at ahigh draw ratio.

The porous material of PTFE according to the present invention combinesmicropores with a high porosity and moreover has a smooth surface highin evenness, high mechanical strength, non-adhesiveness, low frictionalproperties and good flexibility. The porous material permits thepermeation of gases, liquids, fine particles and the like. The porousmaterial of PTFE according to the present invention has a wide varietyof applications such as filter media, membranes or septa, lubricatingmaterials, and non-adhesive materials. In particular, it can be used asa filter for chemicals, a separation membrane for plasma components, aseptum for an artificial lung or the like in fields of semiconductors,medical care, biological industries and the like by making good use ofthe feature that it combines micropores with a high porosity.

EXAMPLES

The present invention will hereinafter be described in detail by thefollowing examples and comparative examples. However, the presentinvention is not limited to these examples only.

Incidentally, the measuring methods of physical properties in thefollowing examples and comparative examples are as follows:

IPA Bubble Point (kg/cm²)

Measurement was conducted with isopropyl alcohol in accordance withASTM-F-316-76.

Porosity (%)

Measurement was conducted in accordance with ASTM-D792.

IPA Flow Rate (ml/cm² /min)

Measurement was conducted with isopropyl alcohol under a differentialpressure of 1 kg/cm².

Retention (%)

A disk 47 mm across was punched out of a sample membrane and set in aholder. On the other hand, an aqueous solution containing uniformparticles of a polystyrene latex sphere (product of Dow Chemical Co.)having a particle diameter of 0.109 μm in a concentration of 1.4×10¹⁰particles/cm³ was prepared. The aqueous solution in an amount of 32 cm³was filtered through the sample membrane set in the holder under apressure of 0.42 kg/cm², thereby determining a retention of theparticles. The retention of the particles was measured by means of aspectrophotometer for ultraviolet and visible region, UV-160manufactured by Shimadzu Corporation at a wavelength of 310 nm toevaluate it. Accuracy in measurement was 1/100.

Another retention of particles was also measured in the same manner asdescribed above except that uniform particles of a polystyrene latexsphere having a particle diameter of 0.073 μm was used.

Example 1

A blend obtained by blending 100 parts by weight of fine powder of apolytetrafluoroethylene (CD-4, product of Asahi Glass Co., Ltd.;molecular weight: 500,000) with 18 parts by weight of Drysol as alubricant was preformed and then extruded into a sheet. This extrudatewas further rolled and then passed through heating rolls to remove thelubricant therefrom, thereby forming a sheet having a thickness of 0.3mm.

This sheet was continuously passed through an heating oven controlled at350-500° C. to sinter it. The sheet thus sintered was then rolled up byabout 60 m around a roll (diameter: 30 cm). The rolled sheet was heldfor 20 hours in an oven controlled at 315° C. and then rewound. Thesheet was then stretched by 200% in an extrusion direction at a rolltemperature of 150° C., and then stretched by 700% in a directionperpendicular to the extrusion direction at an oven temperature of 70°C.

The crystallinity of the PTFE sheet subjected to the heat treatment for20 hours at 315° C. increased to 87%. The crystallinity was determinedfrom the general relationship between the crystallinity and specificgravity of PTFE. The properties of the porous PTFE sheet thus obtainedare shown in Table 1.

Example 2

A heat-treated sheet was obtained in the same manner as in Example 1except that the heat-treating conditions for the sintered sheet werechanged to 315° C. and 10 hours, and a sheet having a crystallinity of83% was provided. The heat-treated sheet was then stretched by 200% inan extrusion direction at a roll temperature of 150° C., and thenstretched by 550% in a direction perpendicular to the extrusiondirection at an oven temperature of 70° C. The properties of the porousPTFE sheet thus obtained are shown in Table 1.

Example 3

A heat-treated sheet was obtained in the same manner as in Example 1except that the heat-treating conditions for the sintered sheet werechanged to 320° C. and 20 hours, and a sheet having a crystallinity of90% was provided. The heat-treated sheet was then stretched by 200% inan extrusion direction at a roll temperature of 150° C., and thenstretched by 550% in a direction perpendicular to the extrusiondirection at an oven temperature of 70° C. The properties of the porousPTFE sheet thus obtained are shown in Table 1. Besides, a scanningmicrophotograph of this porous PTFE sheet is illustrated in FIG. 1.

Example 4

A heat-treated sheet was obtained in the same manner as in Example 1except that the heat-treating conditions for the sintered sheet werechanged to 310° C. and 10 hours, and a sheet having a crystallinity of80% was provided. The heat-treated sheet was then stretched by 100% inan extrusion direction at a roll temperature of 150° C., and thenstretched by 400% in a direction perpendicular to the extrusiondirection at an oven temperature of 70° C. The properties of the porousPTFE sheet thus obtained are shown in Table 1.

Example 5

A porous PTFE sheet was produced in the same manner as in Example 1except that fine powder of another polytetrafluoroethylene (CD-1,product of Asahi Glass Co., Ltd.; molecular weight: 1,000,000-2,000,000)was used. The properties of the porous PTFE sheet thus obtained areshown in Table 1.

Example 6

A porous PTFE sheet was produced in the same manner as in Example 1except that fine powder of another polytetrafluoroethylene (CD-1,product of Asahi Glass Co., Ltd.; molecular weight: 1,000,000-2,000,000)was used, and the amount of Drysol was changed to 22 parts by weight.The properties of the porous PTFE sheet thus obtained are shown in Table1.

Comparative Example 1

A sintered sheet (crystallinity: 76%) was formed in the same manner asin Example 1, stretched by 200% in an extrusion direction at a rolltemperature of 150° C. without conducting any heat treatment, and thenstretched in a direction perpendicular to the extrusion direction.However, the stretching in the perpendicular direction was unable to beperformed only by 300% or lower. The properties of the porous sheetobtained at that time were as shown in Table 1.

Comparative Example 2

A dry sheet having a thickness of 0.3 mm was obtained by the pasteextrusion of the fine powder of the polytetrafluoroethylene in the samemanner as in Example 1. This sheet was heated to 350° C. to sinter it,and then cooled to 300° C. at a cooling rate of 1° C./hr. Thereafter,the thus-cooled sheet was cooled to room temperature in an airatmosphere (25° C.). The sintered sheet thus obtained was stretched by200% in a lengthwise direction and by 400% in a crosswise direction at150° C. The properties of the porous PTFE sheet thus obtained are shownin Table 1.

Comparative Example 3

A blend obtained by blending 100 parts by weight of fine powder of apolytetrafluoroethylene (CD-123, product of Asahi Glass Co., Ltd.;molecular weight: 10,000,000) with 18 parts by weight of Drysol as alubricant was preformed and then extruded into a sheet. This extrudatewas further rolled and then passed through heating rolls to remove thelubricant therefrom, thereby forming a sheet having a thickness of 0.3mm. This sheet was stretched by 200% in a lengthwise direction and by700% in a crosswise direction at a temperature of 150° C. After thestretching, the stretched sheet was heat-treated at 300° C. Theproperties of the porous PTFE sheet thus obtained are shown in Table 1.Besides, a scanning microphotograph of this porous PTFE sheet isillustrated in FIG. 2.

                                      TABLE 1    __________________________________________________________________________            Example            Comp. Ex.            1  2  3  4   5  6  1   2  3    __________________________________________________________________________    Porosity            81.5               85.0                  87.2                     60.3                         60.5                            67.2                               51.4                                   60.5                                      88.0    (%)    IPA bubble            5.8               6.0                  3.8                     5.6 6.1                            5.2                               6.2 5.5                                      3.8    point    (kg/cm.sup.2)    IPA flow            1.8               1.2                  4.1                     0.7 0.6                            0.7                               0.68                                   0.71                                      3.8    rate    (ml/cm.sup.2 /min)    Retention:    0.109 μm            100               100                  100                     100 100                            100                               81  70 55    0.073 μm            50 60 30 70  85 75 --  -- --    __________________________________________________________________________

INDUSTRIAL APPLICABILITY

According to the present invention, a porous material of PTFE farexcellent in retention performance can be provided. According to theproduction process of the present invention, a porous material of apolytetrafluoroethylene, which has micropores and a high porosity andpossesses excellent permeability, can be provided. This porous materialof the polytetrafluoroethylene can be used as a separation membrane, aseptum for an artificial lung or the like in a wide variety of fields ofsemiconductors, medical care, biological industries and the like.

We claim:
 1. A process for the production of a porous material of apolytetrafluoroethylene, which comprises, after forming an article fromfine powder of the polytetrafluoroethylene by paste extrusion, (1)sintering the molded article, (2) rolling up the molded sintered articleand heat-treating it at a temperature lower than a melting point of thepolytetrafluoroethylene in a state as rolled up, wherein the heattreatment is performed at a temperature of 280-325° C. for 1-50 hoursunder substantially isothermal conditions and then (3) stretching themolded heat-treated article in at least an uniaxial direction whereinthe stretching is performed at a draw ratio of at least 9 times.
 2. Aproduction process according to claim 1, wherein an article in the formof a sheet is formed by the paste extrusion of the fine powder of thepolytetrafluoroethylene.
 3. A production process according to claim 1,wherein in the step (2), the heat treatment is performed until thecrystallinity of the polytetrafluoroethylene reaches at least 80%.
 4. Aproduction process according to claim 1, further comprising the step ofconducting heat setting by holding the heat-treated article stretched ata temperature of 150-250° C. for 1-30 minutes and under tension.